The invention relates to laminates of reinforced polyester bonded to metal sheets.
Electrical circuit boards are prepared by laminating metal sheets, e.g. copper sheets, with sheets of electrical insulating materials, such as glass fiber reinforced unsaturated polyester resin sheets. Such electrical circuit boards may be either rigid or flexible, and are further classified as single-sided (metal foil on one side of the insulating material only), double-sided (metal foil on both sides of the insulating material), or multi-layered.
The electrical insulating material is a base or support for the metal sheets bonded thereto. The National Electrical Manufacturers Association (NEMA) has established standards for various base materials. Standards for polyester glass-mat sheet laminates have been established by NEMA, viz. Grades GPO-1, GPO-2, and GPO-3.
Normally the metal foil, usually copper foil, is secured to the reinforced plastic sheet by means of a suitable adhesive. For example, U.S. Pat. No. 3,700,538 discloses adhesive bonding copper foil to resin-impregnated fiberglass cloth using polyimide resin adhesive. It is also known to add an adhesion promoter to the insulating base material. For example, U.S. Pat. Nos. 3,477,900 and 3,149,201, disclose that when the insulating base material comprises methylmethacrylate resin, then unsaturated polyester may be added to the resin as an adhesion promoter to bond a copper foil. However, these patents disclose an increase in the proportion of polyester is generally accompanied by a decrease in adhesion of the copper foil to the resinous base. As the proportion of polyester is increased beyond 45 parts by weight, per 100 parts of methylmethacrylate resin, the adhesion falls rapidly to an unacceptable value. U.S. Pat. No. 4,093,768 discloses that unsaturated polyester resin containing up to about 2% by weight of benzotriazole, incorporated into the resin as an adhesion promoter, can be bonded under pressure directly on a copper foil.
Alternatively, treatment of the surface of the metal foil has been suggested to promote adhesion. U.S. Pat. No. 3,674,445 discloses that a tightly adherent organic coating can be provided on a copper surface by interposing a vacuum vapor deposited zinc coating between the copper surface and the organic coating. U.S. Pat. No. 3,729,294 describes a method of promoting adhesion of a polymer insulating coating to a copper surface by forming a thin film of zinc on the surface and heating the surface to diffuse the zinc therein, to obtain a zinc-diffused surface having the golden appearance of brass. As can be seen, these prior methods are generally complex and therefore expensive to practice.